Nikon D70, What Lense??

Just started to really get into aircraft photography after being a spotter for 20+years

My kit consists of

2 x Nikon D70s
1 x 170-500mm APO lense
2 x 70-300mm lenses

I have heard how good the cannon 100-400 lense is..so what comparison can i get for my Nikon..i have in mind the Sigma 80-400mm F/4.5..but on reading reviews it’s not very quick on the AF side..any advice appreciated
BTW i have found a price of just over £500 for the sigma lense not bad